On 17/06/2011 09:42, Jochen Topf wrote:
The source tag can sometimes be some help in figuring out the history
of an
object. After it was entered in OSM we have a complete history, before that the
source tag can sometimes help. But it is far less useful in practice than many
think. Exactly because the data almost always is a mix of many sources. In many
cases source tags on objects are even misleading, because people don't change
them when they change data, so the data only shows one source when it has many.
Its better to not have them at all.
I'd agree that people sometimes don't update source tags on objects;
just recently a non-local armchair mapper updated a bunch of stuff
locally and did exactly that (they apparently put a source tag on a
changeset, but as that's not visible against the object that's not
actually useful).
I'd disagree that "it's better to not have them at all" though - the
more information about what's there and who added it the better. When I
create Garmin maps I currently do the following:
o Ways and nodes from old out of copyright maps are labelled as such, so
that e.g. footpath data inferred from there can be updated with actual
rights of way
o Ways and nodes added with a remote source or by prolific tracers from
OSSV and Bing are labelled as such, because any "on the ground" stuff
will have been missed there
o Ways and nodes added by, er, "users with historical accuracy problems"
are marked as such so that they can be explicitly checked.
So yes, sources get combined (and you'll see lots of semicolons in
source tags from me reflecting that) but personnally I find source tags
on OSM items extremely useful.
Cheers,
Andy
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